Polishing apparatus

ABSTRACT

The polishing apparatus can control the attitude of the top ring with respect to a surface of a turntable of a polishing apparatus is controlled so as to provide a uniform polish surface pressure across the entire polish surface. The polishing apparatus includes the turntable having an abrading surface, a top ring for holding an object to be polished to keep the object surface in moving contact with the abrading surface while rotating the turntable and the top ring, a magnetic bearing assembly for supporting a rotation shaft of the top ring by means of a thrust bearing device and at least one radial bearing device, and an attitude controller for controlling an orientation of the top ring with respect to the turntable through the magnetic bearing assembly.

This is a continuation of application Ser. No. 08/864,906, filed May 29,1997, now U.S. Pat. No. 5,951,368.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polishing apparatus, and relates inparticular to a polishing apparatus having a top ring or pedestal havinga rotation shaft is supported by a magnetic bearing.

2. Description of the Prior Art

In recent years, there has been a remarkable progress in the density ofintegrated circuit devices leading to a trend of narrowing interlinespacing. In the case of using an optical lithography process involvingless than 0.5 μm line spacing particularly, the shallow depth of focusis associated to demand that the focusing plane of the stepper device behighly flat. Furthermore, if there is a particle of a size larger thanthe interline spacing, problems of electrical short circuiting canoccur. Therefore, both flatness and cleanliness are importantconsiderations in device fabrication. Such considerations apply equallyto glass substrates used in masking or to liquid crystal display panels.

A conventional polishing apparatus, shown in FIG. 19, comprises aturntable 2 having a polishing device (polishing pad) 1 mounted on thetop surface and a top ring 3 holding an object to be polished such assemiconductor wafer W. The turntable 2 and the top ring 3 are providedwith their own rotation drivers so as to be independently rotated, alongwith a pressing device such as an air cylinder to press the wafer ontothe polishing pad.

Such a polishing apparatus is operated by placing the top ring 3 in sucha way that the edge of a wafer W is positioned at a certain distanceaway from the center and the edge of the turntable 2, and by pressingthe wafer W towards the turntable while rotating the turntable 2 and thetop ring 3 at independent speeds and supplying a polishing solution Qfrom a nozzle 4. The aim is to polish the entire surface of the wafer Wuniformly.

The top ring 3 is a disc shaped member holding object W on its undersideholding surface by suction, for example, and it is attached to itssupport shaft 5 at a joint section providing universal coupling by meansof a sphere 6. This structural arrangement allows tilting of the topring 3 in response to the force exerted by the turntable 2 so as tocompensate for any misalignment between the top ring 3 and the turntable2 or local variations in the polishing pad 1 so that polishing can becarried out consistently.

However, such a conventional polishing technique presents a problem thata pressing pressure between the abrading surface (pad surface) of theturntable 2 and the object W tends to be non-uniform across the surfaceso that it is difficult to obtain a uniform removal rate across thesurface of the object W. This will be examined more closely below interms of the relative motions of the object W held in the top ring 3 andthe polishing pad 1 on the turntable 2.

The amount of material removed by polishing is given by a relation:

Qp=η×P×V×T

where Qp is removal rate; ηis a constant; P is a polishing pressure; Vis a relative speed (between object surface of the wafer and theturntable); and T is a polishing duration. Therefore, applying uniformpressure in the polishing area is one of the important factors to obtaina uniform removal rate within the polished surface.

However, as shown in a schematic drawing presented in FIG. 20A, there isa force of friction “f” acting at the polish surface (given by f=mNwhere N is the load on the object W and m is the coefficient offriction) which generates a rotational moment M around the sphere 6.This arrangement produces tilting of the top ring 3, as illustrated inFIG. 20B, resulting in a phenomenon of “plunging” of the leading edgeportion of the object W into the surface layer of the polishing pad 1 asillustrated in FIG. 20C. As shown in FIG. 20B, the this angle θoftilting is actually determined by the action of the sphere 6 accordingto a relation between the pressing force N and the frictional force f.

As shown in FIG. 20D, tilting of the top ring 3 produces a non-uniformdistribution of polish surface pressure so as to cause the pressure atthe edge portion to be higher than in the rest of the object W. Becausethe object W is also rotated, the removal rate distribution becomes onethat is illustrated in a graph shown in FIG. 20E. The polish surfacepressure to obtain a uniform material removal rate across the objectsurface of the object W, is also influenced by the softness of the padand the flow conditions of the polishing solution. Therefore, angle θshould not nessarily be equal to zero. However, the angle θ which isdetermined as the result of reaction of the sphere 6 to various forcesacting on the top ring 3, as mentioned above, is not necessarily theoptimum angle which would produce uniform polish surface pressure.

Also, according to the conventional technique, since the tilt angle θ isdetermined by a result of the reaction of the sphere 6, the possibilityexists that consistent polishing is not produced due to local variationsin the surface conditions existing at the polish surface of thepolishing pad, which may lead to vibration of the top ring to causefurther unsteadiness. As a result, as shown in FIG. 20E, more materialis removed from the peripheral region than in the central region of thewafer.

Also, in recent years, an alternative type of apparatus has beenproposed, which comprises a cylindrical rotating drum having an outerabrading surface. The apparatus applies a line pressure on the wafer,provided by a line contact of the outer periphery of the cylindricalrotating drum with the polished surface of the wafer. While the drum andthe wafer are made to undergo relative movement, a polishing solution issupplied to the contacting surfaces to produce a mirror polishedsurface.

Such rotating drum type apparatus enables the use of more compactpolishing tools, compared with the turntable type apparatus, and as aresult, the polishing apparatus can also be made more compact. Also,because this approach makes it possible to directly observe thecondition of the surface being polished, the amount of material removedor the film thickness remaining on the wafer being polished can bedetermined on a real time basis.

In the drum type apparatus, the workpiece and the polishing tool aremoved relative to each other during the polishing process so as toobtain uniform material removal over the entire object surface of theworkpiece. However, the contact length between the polishing tool andthe workpiece can vary while undergoing such relative movement; forexample, in polishing a circular object such as a semiconductor wafer,the pressing pressure becomes higher in the outer peripheral region ofthe wafer, resulting in an increased removal rate, to cause a so-called“turned-down edge” phenomenon.

This phenomenon is caused by the fact that the apparatus is operatedunder a constant pressing load. Also, pressing devices, such as fluidoperated cylinders, are not suitable for controlling the pressingpressure because of insufficient response speed and precision due to thepresence of fluid pressure adjusting valves, the volumetric effect ofthe cylinder device and the sliding parts between the cylinder pistonand the pressure seals.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a polishingapparatus to enable control of the attitude of the top ring with respectto the surface of the turntable so as to provide a uniform polishsurface pressure across the entire polish surface.

A second object of the present invention is to provide a rotating drumtype polishing apparatus to obtain its mechanical advantages whileimproving its performance disadvantages such as non-uniform pressingpressure over a given shape of an object surface.

The first object is achieved by a polishing apparatus for polishing anobject surface of an object comprising: a turntable having an abradingsurface; a top ring for holding the object to keep the object surface inmoving contact with the abrading surface while rotating the turntableand the top ring; a magnetic bearing assembly for supporting a rotationshaft of the top ring by means of a thrust bearing device and at leastone radial bearing device; and an attitude controller for controlling anorientation of the top ring with respect to the turntable by themagnetic bearing assembly.

Accordingly, by controlling the attitude of the top ring holding theobject with respect to the turntable by means of a magnetic bearingdevice, polishing is carried out under a uniform distribution of polishsurface pressures so as to provide a highly flat polished surface.

If the abrading surface is a grinding stone which has negligibleelasticity, the object surface and the abrading surfaces should beperfectly parallel during the polishing process to obtain good results.If the polishing surface contains soft material such as a polishing pad,then a suitable tilt angle, depending on the polish surface pressure andrelative polishing speeds, should be determined.

Usually, the tilt angle along the moving direction of the turntable iscontrolled. This tilt angle should be determined so as to avoid the“plunging” phenomenon, or to cancel the force generated by the“plunging” phenomenon so that the polish surface pressure distributionis uniform. The tilt angle can be controlled by output signals fromdisplacement sensors of thrust and radial magnetic bearing devices inthe magnetic bearing assembly.

In the present invention, the polish surface pressure can be controlledso that, in addition to controlling the distribution of the polishsurface pressure, it is also possible to control the magnitude of thepressure itself at a proper value. The polish surface pressure isestimated from changes in the load current through the bearings. Bycontrolling the excitation current to the thrust bearing, the polishsurface pressure may also be controlled. An elevator device may also beused to control the polish surface pressure. The polish surface pressurecan be pre-determined by trial polishing operations.

Another embodiment of the present invention is a method for producing apolished surface on an object by a polishing apparatus having aturntable with an abrading surface, a top ring for holding the object tokeep an object surface in moving contact with the abrading surface whilerotating the turntable and the top ring, and a magnetic bearing assemblyhaving a thrust bearing device and at least one radial bearing device soas to magnetically suspend a rotation shaft of the top ring, wherein themethod includes optimizing an attitude of the top ring with respect tothe turntable by control of the magnetic bearing assembly.

The second object of the present invention is achieved by a polishingapparatus comprising: a drum that is rotatable and having a polishingsection mounted on an outer surface of the drum; a pedestal memberholding an object surface of an object to face towards the drum; apressing device for pressing the drum relatively to the object; adriving device for rotating the drum; a moving device to relatively movethe drum and the pedestal member in a plane parallel to the objectsurface, wherein the pressing device is provided with a magnetic bearingdevice for rotatably supporting the pedestal member.

Accordingly, by simply altering electric current flowing in a coil ofthe magnetic bearing, polishing parameters, such as pressing pressureand tilting between the drum and the polishing object, can be controlledquickly and accurately, thereby making it possible to retain the highlyresponsive character needed for a drum type polishing apparatus and toexercise precise control over the pressing pressure and/or orientationof the drum with respect to the object. Improvements in polishingperformance have thus been attained while retaining the advantages ofcompactness, ease of maintenance and visual observation of the polishingprocess of the drum type polishing apparatus. The pressing device alsomay be provided for pressing the drum relative to the object. In thiscase, the pressing device may be provided with a magnetic levitationdevice for linearly movably supporting the drum, as well as a magneticbearing device for rotatably supporting the drum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a polishing apparatus of afirst embodiment according to the present invention.

FIG. 2 is a cross sectional view of a magnetic bearing device shown inFIG. 1.

FIG. 3A is a sectional view along a line 3A—3A in FIG. 2.

FIG. 3B is a sectional view along a line 3B—3B in FIG. 2.

FIG. 4 is a block diagram of a control section for controlling theoperation of a top ring.

FIG. 5 is a perspective view showing tilt parameters α, β with respectto X-, Y- and Z-axes of a rotation shaft.

FIG. 6 is a flowchart for controlling the attitude of the polishingapparatus.

FIG. 7 is a flowchart for controlling the polish surface pressure of thepolishing apparatus.

FIG. 8 is a graph showing variations in the polish surface pressure withduration of polishing before and after the application of a polishsurface pressure control operation.

FIG. 9A is a graph showing the distribution of removal amount across theentire region of a wafer after being polished without setting a propertilting angle.

FIG. 9B is a graph showing the distribution of a removal amount acrossthe wafer after being polished by setting a proper tilting angle.

FIG. 9C is a plan view showing surface roughness measurement positions.

FIG. 10 is a vertical cross sectional view of a second embodiment of thepolishing apparatus of the present invention.

FIG. 11 is a block diagram of an example of a control device to controlthe polishing apparatus.

FIG. 12 is a schematic representation of the control device shown inFIG. 11.

FIG. 13 is an illustration of the operation of a level-holding device.

FIG. 14 is a vertical cross sectional view of another embodiment of thepolishing apparatus.

FIG. 15A is a vertical cross sectional view of another embodiment of thepolishing apparatus.

FIG. 15B is a horizontal cross sectional view, along line 15B—15B ofFIG. 15A, of another embodiment of the polishing apparatus.

FIG. 16A is a perspective view of operation of a drum type polishingapparatus.

FIG. 16B is a cross sectional view operation of the drum type polishingapparatus.

FIG. 16C is a perspective view of a contact interface C where the wafermeets the abrading surface of the drum in a drum type polishingapparatus.

FIG. 17A is an illustration of operation of a pedestal moving devicebased on transverse movements of the wafer.

FIG. 17B is an illustration of operation of the pedestal moving devicebased on a combination of transverse and lateral movements.

FIG. 18A is an illustration of the relation between a contact length Land a drum position on an object to be polished.

FIG. 18B is a graph portraying drum position x and contact length L.

FIG. 19 is a schematic illustration of a conventional polishingapparatus.

FIGS. 20A˜E illustrate various effects introduced by the conventionalpolishing apparatus shown in FIG. 19.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, preferred embodiments of the polishing apparatus willbe presented with reference to the drawings.

FIG. 1 shows an embodiment of the polishing apparatus comprising aturntable 2 having a polishing pad 1 or a grind stone mounted on its topsurface, and a top ring 3 for holding an object to be polished(semiconductor wafer) W. The turntable 2 has a driving device 36 forrotating the turntable independently of the top ring.

The top ring 3 is supported by a magnetic bearing assembly 10, where arotation shaft 12 of the top ring 3 is housed in the interior of acasing 11, and the following devices are provided to support therotation shaft 12 non-contactingly. Such devices are, from the top: athrust bearing 13, an upper radial bearing 14; and a lower radialbearing 15. A thrust displacement sensor 17 (FIG. 2) is provided todetect vertical displacement of the rotation shaft 12, and radialdisplacement sensors 18, 19 are provided to detect displacements in thehorizontal directions. A drive motor 20 is disposed between the upperradial bearing 14 and the lower radial bearing 15, and the magneticbearing assembly 10, including the drive motor 20, is supported by anexternal elevator device 36 (shown in FIG. 4) attached to the casing 11through the support arm 16. The elevator device may be in a form of afeed screw having a precision positioning capability or an air cylinderarrangement.

FIG. 2 shows an enlarged view of the top ring shown in FIG. 1. Themagnetic thrust bearing 13 comprises a thrust disc 13 a fixed on therotation shaft 12; and electromagnetic cores 13 b, 13 c having a pair ofupper and lower coils 21 a, 21 b and fixed to the casing 11. The upperand lower radial bearings 14, 15 comprise electromagnetic cores 14 a, 15a, each having eight magnetic poles 14 c, 15 c extending from the insidesurface of the casing 11 towards the interior of the magnetic device 10and rotor magnetic poles 14 b, 15 b fixed on the rotation shaft 12.Adjacent pairs of the electromagnetic cores 14 a, 15 a across the X- orY axis share a common coil 22, 23, respectively.

The structural configuration of the radial bearing device will beexplained with reference to FIG. 3A, which is an orthogonal crosssectional view relative to the rotation shaft 12. The stator of theradial bearing 14 comprises a pair of electromagnetic coils comprising acoil 22 a disposed in the positive direction of the X-axis, and anopposing coil 22 b disposed in the negative direction of the X-axis. Therotor of the radial bearing 14 comprises a rotor magnetic pole 14 b. Thecoils 22 a, 22 b are individually excited, and a balance in theattractive electromagnetic forces governs the position in theX-direction. Similarly, a pair of electromagnet coils 22 c in theY-direction and 22 d in an opposite Y-direction are provided. The lowerradial bearing 15 is also provided with an electromagnetic core 15 ahaving eight magnetic poles 15 c and their respective coils 23 a, 23 b,23 c and 23 d, and a rotor magnetic pole 15 b fixed on the rotationshaft 12.

As shown in FIG. 3B, the radial displacement sensor 18 has a pair ofradial displacement sensors 18 x to detect movement in the X-directionand a pair of radial displacement sensors 18 y to detect movement in theY-direction. The drive motor 20 has a motor core 20 a, a coil 20 b and amotor rotor 20 c.

FIG. 4 is a block diagram of a controller section for controlling theoperation of the polishing apparatus. The controller section includes asubtracter 30 and a controller 31. The subtracter 30 receives as inputdata a reference value for the position of the control object (e.g.,rotation shaft 12), and current displacement values x, y, z, α and β ofthe control object, generated in sensor section 34 (including radialdisplacement sensors 18, 19 and thrust displacement sensor 17) andconverted in coordinates conversion section 35. The subtracter 30outputs differential signals e_(x), e_(y), e_(z), e_(α), e_(β) from thereference value and the current values which are then entered into thecontroller 31. As shown in FIG. 5, X, Y and Z refer to displacementsalong the X-, Y- and Z-axes, respectively, and α and β are tilt angleswith respect to the X- and Y-axes, respectively.

With reference to FIG. 4, the differential signals e_(x), e_(y), e_(z),e_(α), e_(β) are processed in PID+local phase progression section 31-1to undergo tilt/position controls and an attenuation process, areforwarded to pass through notch filter section 31-2 to filter outvibrational effects, and are converted into electrical voltage commandsignals v_(x), v_(y), v_(z), v_(α), v_(β). These signals are then inputinto coordinates conversion section 31-3 for conversion to controlsignals v_(xu), v_(yu), v_(x1), v_(y1), v_(z) and are input intomagnetic bearing driver section 32 for the radial bearings 14, 15 andthe thrust bearing 13. Signal v_(z) is also input into the polishsurface pressure controller 23.

The magnetic bearing driver section 32 includes electromagnetic coils 22a, 22 b, 22 c, 22 d, 23 a, 23 b, 23 c, 23 d, 21 a, and 21 b and drivingcircuits 24 for exciting these coils. The signals v_(xu), v_(yu),v_(x1), v_(y1), v_(z) are input into respective driving circuits 24, andare converted into magnetic excitation current data I_(xu+), I_(xu−),I_(yu+), I_(yu−), I_(x1+), I_(x1−), I_(y1+, I) _(y1−) to movement in thepositive/negative directions for the radial bearings 14, 15; and I_(z+)and I_(z−) for vertical movement of the thrust bearing 13; and aresupplied to the respective magnetic coils 22 a, 22 b, 22 c, 22 d, 23 a,23 b, 23 c, 23 d, 21 a, and 21 b so as to control the attitude of thecontrol object (e.g., rotation shaft 12).

The polish surface pressure controller 23 provides a fine control of thepressing force exerted by the top ring 3 against the turntable 2, andreceives current control signals in the Z-direction, as well as inputsignals from the sensor section 34. The signals from the polish surfacepressure controller 23 are output as pressing force control signals inthe Z-direction, and are added to a signal V_(z) by an adder 22 whichfollows the controller 31. The pressing force control signals from thepolish surface pressure controller 23 is also input to the elevatordevice 36.

Next, a method of controlling the operation of the polishing apparatuswill be explained with reference to flowcharts shown in FIGS. 6 and 7.First, the steps for adjusting the attitude of the top ring 3 will beexplained with reference to FIG. 6. In the beginning, empirical initialparameters are entered (st1) and then trial polishing is performed(st2). Next, uniformity of the polished surface is evaluated by asuitable technique, and the results are examined to determine whethercertain conditions for uniformity are satisfied (st3) If the conditionsare not met, the parameters are re-entered (st4), and trial polishing isrepeated. If a certain angle is found to satisfy the uniformityconditions, production polishing is carried out (st5) using this valueof the parameter as a reference value.

An example of evaluating the flatness uniformity on the basis of surfaceroughness measurements is shown in FIGS. 9A-9B. FIG. 9A relates to acase of polishing without setting the tilt angle, and FIG. 9B relates toa case after setting the tilt angle to an optimum value. It can be seenthat the optimum angle of tilt angle produces better flatness. FIG. 9Cshows the positions where the measurements were taken. In the trialpolishing, the parameters are set to be identical to those in an actualproduction polishing operation. Typical parameters are the types ofobject being polished and polishing solution, rotational speeds of theturntable and top ring, and polish surface pressure, for example.

As described above, by setting the tilt angle of the rotation shaft 12at a proper value, it is controlled by functions of the magnetic bearingassembly, i.e., by sensing the tilt angle from the output signals fromthe radial displacement sensors 18, 19 and adjusting the tilt of therotation shaft 12 suitably. Thus, the polish surface pressure betweenthe object surface and the abrading surface can be controlled uniformlyacross the area of contact.

Next, the steps of controlling the polish surface pressure will beexplained. An optimum value of the polish surface pressure is alsopre-determined by a trial polishing, and this reference value is enteredas a reference value. As shown in FIG. 1, the elevator device 36 appliesa vertical force on the casing 11, which is eventually transferred tothe polish surface through the thrust bearing 13, rotation shaft 12 andthe top ring 3. Here, the thrust bearing 13 is subjected to a reactionforce of the polish surface pressing force. Thus, if a pressurevariation is generated at the polish surface, this variation can bedetected by the current flowing in the thrust bearing 13.

The relation between the pressing force F and the current through thecoil 21 a, 21 b can be approximated by the following formula, where K isa coefficient, I a current, and F is a force:

F=KI.

F is computed from the above relation, and the pressing force values arecompared. Adjustment values are forwarded to the elevator device 36 toadjust the pressure. It is also possible to adjust the current of themagnetic bearing assembly 10 itself to adjust the polish surfacepressure. In this case, the control current through the thrust coils 21a, 21 b is used as the parameter for the polish surface pressurecontrol.

The polish surface pressure control steps for adjusting the polishsurface pressure will be explained with reference to FIG. 7. The polishsurface pressure controller 23 detects the magnitude of the controlcurrent in the thrust bearing 13 in the driving circuit 24 (st1), andthe polish surface pressure controller 23 computes the force acting onthe rotation shaft 12 according to output data from the displacementsensor 34 and the control current (st2) In this case, if the levitatedposition is unchanged, the value of K in st2 in the flowchart will notchange. The computed polish surface pressure is compared with thereference value (st3), and if the result is not within the allowablerange, a correction pressure is computed (st4). The elevator device 36is operated according to the computed correction pressure, and theposition of the entire bearing assembly 10 is adjusted (st5). Theprocess is returned to st1, and the process is repeatedly performeduntil the difference between the computed and measured values of thepolish surface pressure are in the allowable range. A polishingoperation is carried out through the above described process under apre-determined proper polish surface pressure. The polish surfacepressure variation during such process is illustrated in FIG. 8.

The responsiveness of the control arrangement depends on the drivemechanism of the elevator device 36. If the drive comprises a ball screwmechanism operated by a stepping motor, the adjustment is carried outmechanically and the polish surface pressure is adjusted indirectly, sothat the responsiveness is not too fast and is able to deal with slowlychanging polish surface pressures. Instead of such an elevator device36, it is permissible to control the position of the top ring 3 byapplying the computed value of control current to the thrust bearing 13so as to directly control the operating current value through themagnetic bearing coil 21 a, 21 b, i.e. control of thrust force. Becausethe magnetic control method is more direct than the mechanical controltechnique, the control responsiveness is superior.

Other embodiments according to the present invention will be describedwith reference to the FIGS. 10 to 18B.

As shown in FIG. 10, the polishing apparatus according to the secondembodiment comprises: a base 110; workpiece supporting device 114 tosupport a workpiece (object to be polished) 112; a polishing tool 116for polishing the workpiece 112; and a polishing solution supply pipe(not shown) to supply a polishing solution containing abrasive particlesto the contacting surfaces of the workpiece 112 and the polishing tool116.

The polishing tool 116 comprises a frame member 118 erected on the base110 to support a drum 120 on which a polishing pad or a grinding stoneis mounted, in such a way that the drum axis is horizontal and the drumis rotatable about the axis. The elastic strength of the drum 120 ischosen so that it will not deflect under normal working conditions bybeing supported at both ends in bearings 122. A drum motor 124 isprovided above the frame member 118 for rotating the drum 120 throughreduction gears. The base 110 is fixed on an installation floor throughan unshown leveler so as to be able to adjust the level of base 110.

The workpiece supporting device 114 comprises a pedestal member 128having a shaft member 126 below and holding workpiece 112 above; apedestal motor 132 disposed on an x-y table 130 with the shaft member126 acting as its rotor shaft; and a magnetic bearing device 134 forsupport of the rotating shaft member 126. The x-y table 130 is apositioning table for moving the workpiece 112 in directions transverseand lateral to the drum axis. The pedestal 128 is provided with a vacuumsuction device to firmly hold the workpiece 112 thereon.

The pedestal motor 132 and the magnetic bearing device 134 are assembledinto a cylindrical casing 136 disposed on top of the x-y table 130.Device 134 includes a movable section attached to the outer periphery ofthe shaft member 126 and stationary section assembled inside of thecasing 136. The components are, successively from the bottom, thepedestal motor 132, a lower radial magnetic bearing 138, a thrustmagnetic bearing 140, an upper radial magnetic bearing 142. In thevicinity of each of the radial magnetic bearings 138, 142, radialsensors 144, 146 are respectively provided, and near the x-y table 130opposite to the bottom end of the shaft member 126, an thrust sensor 148is provided. Also, a touch-down or emergency bearing 150 is provideddirectly below the pedestal member 128.

FIG. 11 shows a block diagram of control system 152 for such polishingapparatus. The control system 152 receives signals from radial sensor144, 146; thrust sensors 148; a pedestal displacement sensor 154 whichwill be described later; an x-y sensor 131 for sensing the displacementof the x-y table 130; and a pressing pressure sensor 156 which will bedescribed later. The output signals from the control system 152 areoutput to each of driving circuits for the x-y table 130, pedestal motor132, drum motor 124, radial magnetic bearings 138, 142 and the thrustmagnetic bearing 140.

As shown in an illustration presented in FIG. 12, the following threecontrol functions are performed by the magnetic bearing device 134 whichfreely rotatably levitates the pedestal 128.

(1) Parallelism control: this function controls the horizontalorientation of the pedestal member 128 with respect to the axis of thedrum 120.

(2) Levitation control: this function controls the uplifting of thepedestal member 128 at the start, by gently raising the pedestal member128 toward the drum 120.

(3) Pressing force control: this function controls the pressing force ofthe drum 120 against the workpiece 112 during the polishing operation.

Each of these functions will be described below.

(1) Parallelism Control

As shown in FIG. 13, displacement sensor 154 is installed on each end ofthe drum 120 opposite to the pedestal member 128. The shaft member 126is levitated to a position and then moved in the x- and y-directionsusing the x-y table 130. During such process, the tilt angles θx and θyof the pedestal member 128 area calculated from the stored horizontaldisplacement before and after such movement. From these angles data, thestandard positions of the upper and lower radial bearings 138, 142 inthe x- and y-directions are calculated, and any deviations from thestandard positions are adjusted automatically by generating offsettingposition adjustment values for one or both of the bearings 138, 142 sothat the pedestal member 128 will always be orientated level withrespect to the edge line of the drum 120. By directly measuring thedistance between the drum 120 and the pedestal member 128 to adjust theparallelism therebetween, a uniform contact between the polishing tool116 and the workpiece 112 can always be maintained, even if the axis ofthe drum 120 itself is not level.

(2) Levitation Control

The thrust magnetic bearing 140 enables the pedestal member 128 to belifted gently at the time of shaft levitation operation, to preventpossible damage to the workpiece 112 by sudden impact of the drum 120against the workpiece 112.

To achieve gentle lifting during startup, the pedestal member 128 isfirst lifted temporarily into a position, and then, without setting athrust reference position, the shaft member 126 is slowly liftedaccording to a time-dependent curve as shown in FIG. 12, so that theworkpiece 112 comes into intimate contact with the drum 120. To shortenthe startup time as much as possible, it is preferable that, during theinitial phase, the time-dependent curve may be given a constant steepgradient or an accelerating steep curve, and after the pedestal has comeclose to the reference position, the rise rate is moderated.

(3) Pressing Force Control

To control the pressing force, it is necessary to measure the pressingforce with a sensor or compute the pressing force on the basis ofdisplacement or current data. Once the value of the pressing force hasbeen determined, the input current to the thrust bearing 140 then can beadjusted according to feedback data from the thrust bearing 140.

The pressing force may also be adjusted according to the position of thex-y table 130. For this purpose, the positional referential data of thex-y table 130 is pre-determined and input into the controller, so thatthe pressing force may be adjusted according to a data table containingthe referential value of the pressing force. By adopting this approach,the current can be altered depending on the length of contact betweenthe workpiece 112 and the polishing tool 116 so that the object surfaceis always under a constant pressing pressure.

The pressing force may be controlled in one of two ways by using (a)hardware or (b) software, as explained below.

(a) Using hardware

The electromagnets in the thrust magnetic bearing 140 are affected bythe force of reaction to the pressing force of the drum, and thisreaction force can be measured. The sensor 156 for sensing this reactionforce may be piezoelectric elements or strain gages. These sensors maybe installed between the thrust electromagnets and the spacer.

(b) Using software

The pressing force may be determined from computations, without directlymeasuring the pressing force, from the displacement data in the thrustdirection and the current flowing in the electromagnets in the thrustmagnetic bearing 140. This method can be practiced without changing thestructural configuration of the polishing apparatus, but in order todetermine the pressing force accurately, it is necessary to have firmdata regarding the operating characteristics of the electromagnets (i.e.relation between thrust displacement and current).

The embodiment above was based on a system for levitating the pedestalmember 128 for holding the workpiece 112 by the magnetic bearing device134, however, it is obvious that the electromagnetic support structurecan also be applied to the drum 120. The magnetic support devices mayalso be placed on both the pedestal side and the drum side of theapparatus. The rotation of the pedestal member 128 is carried out bypedestal motor 132 assembled into the magnetic bearing device.

FIG. 14 shows an example of a compact design of the magnetic bearingstructure having an integral type of motor 158 which combines theseparate functions of the lower radial bearing 138 and the pedestalmotor 132 into a single unit.

FIGS. 15A and 15B show another example in which the lower radial bearinghas been eliminated with a combined function bearing 160 which isdisposed at the bottom, and is able to adjust thrust displacement andtilt at the same time so that the tilt and the pressing force can beadjusted adequately.

In any of the control devices presented above, the basic function of themagnetic bearing device 134 is to detect the position of the levitatedobject by sensors, adjust the current in the coils of the electromagnetsaccording to a differential signal between the reference and currentpositions, and maintain the position of the levitated object. Forexample, in FIG. 10, the pedestal member 128 is raised to the referenceposition and held there by the actions of the upper radial magneticbearing 142, lower radial magnetic bearing 138, thrust bearing 140, inassociation with respective sensors 146, 144, and 148, so that theposition of the pedestal member 128 in the radial and thrust directionscan be determined. Therefore, it is possible to maintain the positionand orientation of the pedestal member 128 by specifying a suitablereferential value.

FIGS. 16A-16C illustrate the basic operations of the polishingapparatus. As shown in FIGS. 16A and 16B, the drum 120 having apolishing pad 121 mounted on its outer surface is rotated about its axisto polish the surface of a workpiece 112. As shown in FIG. 16C, thecontact interface is formed along a line. By moving the pedestal member128 in the x-direction with respect to the drum 120, whose axis extendsin the y-direction, the entire surface of the workpiece 112 may bepolished under a uniform pressure.

This arrangement of the polishing apparatus enables the apparatus to beoperated in a relatively small installation space, which willaccommodate the drum 120 and the pedestal moving device. Therefore,compared with the conventional polishing apparatuses, the presentpolishing apparatus becomes significantly more compact and light inweight. Further, because the object surface is visually observable fromabove, the amount of removed material and the thickness of filmremaining on the object surface can always be confirmed during thepolishing operation.

The performance of the pedestal moving device for moving the pedestalmember 128 carrying the workpiece 112 will be explained with referenceto FIGS. 17A and 17B. If the axis of the drum 120 is made stationary,and the movement is based only on a transverse motion (x-direction whichis transverse to y-direction) of the pedestal member 128, surfaceirregularity will be produced when there is uneven pressing pressuredistribution. As shown in FIG. 17A, a combined movement along lateral(y-) and transverse (x-) directions can prevent the formation of objectsurface irregularities. The example shows a case of moving the pedestalmember 128, but the same effects are obtained when the drum 120 is movedin a similar manner.

FIG. 17B illustrates a case of combined swing motions of the rotarysections, i.e. wafer 112 and a sacrificial plate. That is, the pedestalmember 128 can be rotated with the pedestal motor 132 so that anoscillating swing motion of the pedestal member 128 can be achieved.Therefore, the combined movement of the x-y table 130 with the swingmotion of the pedestal member 128 further prevents the formation ofobject surface irregularities.

In general, the removal rate Qp of the material by a polishing processis proportional to: a pressing pressure P at the contacting surfaces ofthe drum 120 and the workpiece 112; a relative speed (or rotational drumspeed) V between the polishing pad and the workpiece 112; and thepolishing duration T. That is, Qp is given by:

Qp=η×P×V×T

as described earlier, where η is a constant of proportionality.

Because polishing is performed in an area of contacting surfaces of thepolishing pad on the rotating drum and the workpiece, when polishing acircular object, such as a semiconductor wafer, the contact length Lchanges as the drum moves over the surface of the wafer. Therefore, whenthe pressing force is kept constant, the pressing pressure changesdepending on the location of the drum on the wafer, leading tonon-uniform across the surface of the polishing object.

In other words, in the central region of the wafer, the contact length Lis long, and in the peripheral region of the wafer, the contact length Lis short. Therefore, when the pressing force is kept constant, thepressing pressure in the peripheral region becomes higher relative tothe central region, and the value of Qp becomes large.

In embodiment of the invention, a remedial approach is taken tocompensate for the varying lengths L by suitably varying the pressingpressure P or drum speed or relative speed V. The contact length L for awafer of radius r, as shown in FIG. 18A, is given by:

L=2(r ² −x ²)^(½).

The value of x is obtained from the value of movement of the x-y table130. The relation between the contact length L and x is expressed as acurve shown in FIG. 18B.

Designating the pressing force by S, the contact pressure P is given by:

P=γS/L=γS/2(r ² −x ²)^(½)

where γ is a constant. Therefore, by controlling the force S such that

S=εL

where ε is a constant, a constant pressure P can be generated regardlessof the contact length L so that uniform polishing can be achieved overthe entire surface of the wafer.

Therefore, by incorporating the distance of movement x of the x-y table130 in the control section, as shown in FIG. 12, computing the value ofL from the distance x, and controlling the current supplied to thethrust magnetic bearing 140, the pressing pressure P between theworkpiece 112 and the polishing pad 121 is:

S=εL=2ε(r ² −x ²)^(½)

where ε is a constant of proportionality, a constant pressure P can beapplied at the contact interface so that the amount Qp of materialremoved remains constant regardless of the value of the contact lengthL.

The rotational speed V of the drum 120 is controlled by supplying arotation speed signal to the drum motor 124 from the control system.Therefore, by maintaining the pressing force S constant, the drum speedV is controlled according to:

V=εL=2ε(r ² −x ²)^(½)

then, because

Qp=η×P×V×T

and

P=γS/L,

a constant value of the removal amount Qp may be obtained regardless ofthe value of the contact length L.

In the above embodiments, the position of the top side of the apparatusincluding the drum 120 was fixed and the lower side of the apparatusincluding the pedestal member 128 with the workpiece was made to move toobtain uniform polishing on the polishing object. However, the lowerside may be fixed in position and the top side may be moved to obtainequal effects.

We claim:
 1. A polishing apparatus for polishing a surface of an objectto a high degree of flatness, said apparatus comprising: a rotatableturntable having an abrading surface; a rotatable top ring for holdingthe object surface against said abrading surface while said turntableand said top ring are rotated; a bearing assembly rotatably supportingsaid top ring, said bearing assembly including at least one bearingdevice operable to change an orientation of said top ring; and anattitude controller connected to said bearing assembly and operable tocontrol said orientation of said top ring and thereby to create an angleof inclination of the object surface relative to said abrading surfaceto be an optimum value to achieve a maximum degree of flatness of thepolished object surface.
 2. An apparatus as claimed in claim 1, whereinsaid bearing assembly comprises a magnetic bearing assembly.
 3. Anapparatus as claimed in claim 1, wherein said abrading surface is formedby soft material.
 4. An apparatus as claimed in claim 1, furthercomprising displacement sensors in said bearing assembly for detectingsaid orientation, and wherein said controller is operable in response todetection by said sensors.
 5. An apparatus as claimed in claim 1,further comprising a polish surface pressure controller for controllinga magnitude of pressure acting between the object surface and saidabrading surface.
 6. An apparatus as claimed in claim 5, wherein saidpolish surface pressure controller is operable to estimate saidmagnitude of pressure as a function of electrical current supplied tosaid at least one bearing device.
 7. An apparatus as claimed in claim 5,wherein said bearing assembly includes a thrust bearing device, and saidpolish surface pressure controller is operable to control said magnitudeof pressure by controlling an excitation current supplied to said thrustbearing device.
 8. An apparatus as claimed in claim 5, furthercomprising an elevator device for raising and lowering said bearingassembly, and said polish surface pressure controller is operable tocontrol said elevator device.
 9. An apparatus as claimed in claim 5,wherein said polish surface pressure controller is operable to controlsaid magnitude of pressure to correspond to a predetermined pressurebased on preliminary trial polishing operations.
 10. An apparatus asclaimed in claim 1, wherein said at least one bearing device comprisesaxially spaced radial bearing devices and a thrust bearing device. 11.An apparatus as claimed in claim 1, wherein said abrading surface is asurface of a polishing pad attached to a polishing pad attachmentsurface of said turntable.